Electrical devices fabricated with semiconductor materials are often characterized by electron current flow and hole current flow which provide the basis for transistor action. Other than the highly important transistor action which can be accomplished by using P and N junctions, other unique electrical effects can be obtained. For example, an electron tunneling effect can be achieved by forming a junction between a highly doped P-type semiconductor material and a highly doped N-type semiconductor material. When so formed, a negative resistance characteristic can be achieved, i.e., as the forward voltage across the junction is increased, the current therethrough decreases. Such negative resistance characteristics have been utilized in amplifiers and other circuits such as mixers, oscillators and detectors. The high-frequency characteristics achieved by the tunneling effect make such devices particularly applicable to microwave circuits.
The tunneling of electrons through a PN junction occurs most effectively when a high electric field exists in a semiconductor, such as within a reverse-biased depletion region of a silicon PN junction. A narrow depletion region can be achieved when the doping level of the P and N-type materials is high, on the order of 1E18 atoms per cm.sup.2, or greater. With such a narrow depletion region, electrons can tunnel therethrough from the P side to the N side, where otherwise such phenomena would not occur. The tunneling (or Zener breakdown) of electrons occurs when the covalent bonds between neighboring silicon atoms become distorted, whereupon some bonds are torn apart, thereby resulting in conduction electrons and holes. The phenomenon in which the conduction electrons penetrate through the energy gap is termed "tunneling." Tunnel transistor operation is described in the text Physics of Semiconductor Devices, by S. M. Sze, published by John Wiley & Sons, Inc. 1981.
While the tunnel effect of currently available two-terminal diodes is highly advantageous in certain applications, such devices are required to be integrated with other devices having other characteristics to achieve an overall circuit function. In addition, the tunnel effect of conventional tunnel diodes cannot be controlled, except by circuits external thereto. Hence, it can be seen that a need exists for a semiconductor device with a terminal for controlling the characteristics of the device. An associated need exists for a semiconductor device which exhibits electrical characteristics, other than the negative resistance characteristic, to thereby enhance the flexibility and application of such device.